A pair of tourists face up to six years in prison after allegedly stealing a large quantity of sand from the pristine beaches of Sardinia.

The French couple were found to have nearly 40kg of fine white sand in the boot of their car.

The vehicle was stopped during a routine check by border police as the tourists were preparing to board a ferry in Porto Torres, bound for Toulon in France.

The sand was found in 14 large plastic bottles and had been taken from a beach near Chia in southern Sardinia.

The couple told police they had no idea they were breaking the law, but now face between one and six years in jail.

The Italian island has battled for years to stop tourists from stealing its sand, shells and pebbles, which are prized as souvenirs, or in some cases for indoor aquariums.

In an attempt to stop the pillaging, some locals have taken on the role of self-appointed guardians of the beaches.

If they see tourists taking sand or shells, they ask them to return the material. If that does not work, they call the police or national park rangers.

One of them, Pina Careddu, said visitors sometimes become rude and aggressive when challenged.

"A family of Germans were filling up some bottles with sand. I recorded them on my phone so they couldn't deny it. The father came towards me in a threatening manner. But in the end, he tipped the sand back onto the beach," Ms Careddu said.